1. Field of the Invention
The present invention relates to a drive control apparatus and a drive control method, and more particularly, to a technique of driving a stepping motor by a microstep driving method.
2. Description of the Related Art
It is possible to control the stepping motor to rotate in steps of particular angles by switching currents passed through coils. Therefore, it is possible to control a position without using a position sensor. Therefore, an open loop control is widely used as a method of controlling the stepping motor. In the open loop control, states of coils in terms of currents flowing through the coils are switched at particular time intervals. However, in a case where the motor is driven at a high speed or in a case where an abrupt change occurs in load on the motor, there is a possibility that it becomes impossible for a rotor to follow the switching in the currents flowing through the coils, which may cause step-out to occur. To handle the above situation, it is known to configure the stepping motor to have a position detection sensor to detect the rotational position of the rotor, and control the stepping motor by a feedback control such that when the speed is increased or decreased, the currents flowing through the coils are switched according to an output from the position detection sensor.
Japanese Patent No. 4165915 discloses a stepping motor that is controlled such that the open loop control is used to drive the motor in a particular period after the driving of the motor is started and in a particular period before the motor is stopped, while the feedback control is used in the other period. This technique allows it to drive the motor at a high speed, and furthermore it is possible to precisely control the motor when the motor is stopped.
However, in a drive control apparatus disclosed in Japanese Patent No. 4165915, an excitation method used to drive the motor by the open loop control is different from an excitation method used to drive the motor by the feedback control. In the open loop control, the motor is driven using microstep driving such that phases of driving currents that are passed through the coils for excitement are changed in steps of small amounts thereby obtaining a sinusoidal-waveform applied voltage. On the other hand, in the feedback control, the motor is driven by applying a rectangular-waveform voltage.
The difference in the excitation method restricts the timing of switching the control from the open loop control to the feedback control, because the units of change in driving current are switched from microsteps to large steps at the transition of the control and this may lead to instability in driving the motor unless the control is switched with correct timing. Thus, to assure the stability in driving the motor, the control is switched after waiting for the correct timing.
The restriction on the timing of switching the control may result in a possibility that the timing of switching the control from the open loop control to the feedback control is delayed from intended timing. The delay in the timing of switching the control may result in an increase in time necessary to reach a target driving position.